1. Field of the Invention
The present invention relates to a method for efficiently recovering energy from off-gas with high temperatures and high pressures released from the oxidation reactor of aromatic compounds such as xylene, pseudocumene, durene and the like.
2. Description of the Prior Art
Conventionally, off-gas released from the liquid phase oxidation reactor of aromatic compounds, in order to recover effective components in the off-gas and to control the reaction temperature, is firstly cooled by heat exchange; then, the condensed components in the off-gas are separated and part of them is circulated to the reactor for reuse while the rest is subjected to recovery of effective components. Meanwhile, the non-condensible off-gas component is preheated and then sent to an expander to recover energy from these off-gas component. [Japanese Patent Application Kokai (Laid-Open) No. 131458/1976, Japanese Patent Model Publication No. 48615/1985]. This prior art is shown in the process flow diagram of FIG. 1. That is, an off-gas 1 released from oxidation reactor 6 with a temperature of about 200.degree. C. and a pressure of 15 to 20 atm is sent to a heat exchanger 2 where low pressure steam is generated by heat exchange; the off-gas after heat exchange is sent to a cooler 3 for cooling; the cooled off-gas is sent to a separator 4 where the condensible components in the off-gas such as the reaction solvent, organic substances as by products and the like are condensed; part of the condensed components is circulated to a reactor 6 via a pipe 5 for recovery; and the rest of the condensed components is taken out via a pipe 7 for suitable disposal. Meanwhile, the off-gas component is sent to a preheater 9 via a pipe 8 for heating; the off-gas component after heating is sent to a combustor 10 where the organic components remaining in the heated off-gas component which may corrode an expander 11 are subjected to catalytic combustion to be converted inorganic substances; and the gas after combustion is sent to the expander 11 to recover energy there from.
In this prior art, however, it is impossible to recover the heat from the off-gas with a temperature of about 150.degree. C. leaving the heat exchanger 2, and this off-gas is then cooled at the cooler 3 to around normal temperature by cooling water to enable recovery and removal of the condensible components in the off-gas at the separator 4. Accordingly, the heat energy in the off-gas leaving the heat exchanger 2 is wasted.
Separately from the above heat recovery from oxidation reactor, the present inventor previously found that in a water injection type regenerative gas turbine cycle, the thermal efficiency can be improved by directly contacting an outlet gas (air) of an air compressor with water to humidify the gas and feeding the mixture of off-gas and steam to a gas turbine. Based on this finding, the inventor filed application for a patent (U.S. Pat. No. 4,537,023).
The present inventor also found a method for efficient recovery of waste heat from processes wherein the waste heat of plants other than gas turbine cycle plants is subjected to heat exchange with cooled water from the bottom of a compressed air-liquid phase water contacting tower and the heated liquid phase water is circulated to the contacting tower. Based on this finding, the inventor filed an application for a patent [Japanese Patent Application Kokai (Laid-Open) No. 106924/1986].